This application for a K23 award combines coursework, a research plan, and personal mentoring to train Anushua Sinha M.D., M.P.H. for independence as a clinical investigator. Dr. Sinha is boarded in Internal Medicine, Pediatrics, and Infectious Diseases. She currently holds an individual NRSA from NIAID to develop and use automated medical record linkage systems to assess the epidemiology of infections during the first 30 days of life. Dr. Richard Platt (Harvard Medical School) and Dr. Milton Weinstein (Harvard School of Public Health) will serve as research mentors. The research builds on Dr. Sinha's NIAID-funded work to investigate the impact of maternal antibiotic prophylaxis against neonatal group B streptococcus on non-GBS infectious outcomes of both infants and mothers. The adoption in the mid-199Os of prophylaxis during labor for approximately 25% of women greatly reduced the occurrence of neonatal GBS disease. However, little is known about its effects on other antibiotic susceptible and resistant infections of mothers and infants, or more generally on the emergence of antimicrobial resistance. It is important to elucidate these issues, and to ascertain patient utilities, to guide decisions about whether and how to use a GBS vaccine that is currently being developed at the investigator's institution (The Streptococcal Initiative, PI: D. Kasper, NO1-AI-75326). The study thus has 5 specific aims: Assess the impact of GBS prophylaxis on 1) neonatal non-GBS infections; 2) maternal non-GBS outcomes, 3) antibiotic resistant infections among prophylaxis recipients, and among newborns and postpartum women generally; and 4) resource utilization. Aim 5 assesses the cost effectiveness of antibiotic prophylaxis compared to immunization. Dr. Sinha will address these issues using direct patient interviews together with data from an existing cohort of 15,532 deliveries from 10/1/90 to 3/31/98 for which she has linked extensive, automated inpatient and outpatient records. Case-control and cohort methods will be used to address aims 14, and decision analysis for aim 5.